4.12.7.2 Example: retract
     | ?- assert(p(1)), assert(p(2)), assert(p(3)).
     
     yes
     | ?- p(N), write(N), nl, retract(p(2)),
          retract(p(3)), fail.
     1
     2
     3
     
     no
     | ?- p(N), write(N), fail.
     1
     no
     | ?-

At the first call to p/1, the procedure has three clauses. These remain visible throughout execution of the call to p/1. Thus, when backtracking is forced by fail/0, N is bound to 2 and written. The retraction is again attempted, causing backtracking into p/1. N is bound to 3 and written out. The call to retract/1 fails. There are no more clauses in p/1, so the query finally fails. A subsequent call to p/1, made after the retractions, sees only one clause.


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